Holograms for displaying visual information are being applied to an ever-widening range of fields including head-up displays, aiming sights, light collimators and light focusing apparatuses.
Previous methods of illuminating a hologram include illuminating the hologram from the rear surface thereof and transmitting the light through the hologram. This type of hologram is commonly referred to as a transmission hologram. The first transmission holograms transmit the reference beam on axis with the object beam. Later developments provide off axis transmission such that an observer views the image without directly looking into the reconstruction beam.
Reflection holograms have been developed which require a front light source. Even though reflection holograms eliminate the need for a light source behind the hologram thereby allowing a hologram to be mounted on a solid wall, much care has to be taken in choosing the appropriate light source angle so that the observer does not obstruct the light source and cast a shadow on the hologram. Furthermore the space in front of the hologram needs to be free of other obstacles that can cast interfering shadows.
Holograms, particularly glass plate holograms, can also be illuminated with an expanded laser beam that enters the side edge of the hologram cover plate. Side edge illuminated holograms are initially recorded with a reference beam entering the side edge of the hologram cover plate. One such side edge illumination system is disclosed in my U.S. Pat. No. 4,643,515 issued Feb. 17, 1987.
Expansion of the laser beams in ambient air has several disadvantages. Firstly, the space required to expand the beam renders the hologram display larger. The beam exiting a laser and travelling through air requires at least one lens, and several air-glass interfaces before the light enters the hologram edge. Each glass surface must be kept clean and free of moisture condensation in order to reconstruct the hologram. Keeping the hologram and each glass surface dry and dust free can be a serious problem in a holographic weapon sight application that is used in the field. Secondly, the laser used for expanding the laser beam in air is an electrically pumped gas or light pump solid laser that is large and often consumes more than 10 watts of input power at voltages of over 1,000 volts. The typical laser occupies space greater than 100,000 cubic millimeters.
Compact lasers such as laser diodes are known. The power consumption of a laser diode is often under 1 watt and operates with less than 5 volts. The volume of a laser diode is under a few cubic centimeters. Present laser diodes however emit a beam that has wavelength drift. The wavelength drift is undesirable for a clear accurate reconstruction of a hologram.
What is needed is a compact hologram display that is lightweight, has low power consumption, reduces exposed air-glass interfaces and sufficiently compensates for the wavelength drift of a compact laser diode. A system with the above advantages is also desired in an edge illuminated hologram display where the reference beam cannot be obstructed and the potential for shadows cast on the hologram is eliminated. Furthermore, a hologram display is desired that does not require extra space for the divergence of the laser beam.